Genotypic variation in calling song and female preferences of the field cricket Teleogryllus oceanicus

Sexual selection: When crickets go quiet

Mutations that change male cricket song should be at a disadvantage because the song is used by females to choose amongst males. A new study caught evolution in action and showed that females may have flexible preferences, and new songs may even be preferred so that the mutations spread.

Spoiled for choice: number of signalers constrains mate choice based on acoustic signals

Animal communication mediates social interactions with important fitness consequences for individuals. Receivers use signals to detect and discriminate among potential mates. Extensive research effort has focused on how receiver behavior imposes selection on signalers and signals. However, animals communicate in socially and physically complex environments with important biotic and abiotic features that are often excluded from controlled laboratory experiments, including noise. “Noise” is any factor that prevents signal detection and discrimination. The noise caused by aggregates of acoustic signalers is a well-known impediment to receivers, but how many individual signalers are required to produce the emergent effects of chorus noise on receiver behavior? In Teleogryllus oceanicus, the Australian field cricket, we assayed female preferences for a temporal property of male advertisement signals, the number of long chirp pulses, using two-, four-, six-, and eight-choice phonotaxis experiments. We found that, as the number of individual signalers increased, receivers became less likely to respond phonotactically and less likely to express their well-documented preference for more long chirp pulses. We found that very few individual signalers can create a sufficiently noisy environment, due either to acoustic interference or choice overload, to substantially impair female preference expression. Our results suggest that receivers may not always be able to express their well-documented mating preferences in nature.

Sexual selection and population divergence III: Interspecific and intraspecific variation in mating signals

A major challenge for studying the role of sexual selection in divergence and speciation is understanding the relative influence of different sexually selected signals on those processes in both intra- and interspecific contexts. Different signals may be more or less susceptible to co-option for species identification depending on the balance of sexual and ecological selection acting upon them. To examine this, we tested three predictions to explain geographic variation in long- versus short-range sexual signals across a 3,500 + km transect of two related Australian field cricket species (Teleogryllus spp.): (a) selection for species recognition, (b) environmental adaptation and (c) stochastic divergence. We measured male calling song and male and female cuticular hydrocarbons (CHCs) in offspring derived from wild populations, reared under common garden conditions. Song clearly differentiated the species, and no hybrids were observed suggesting that hybridization is rare or absent. Spatial variation in song was not predicted by geography, genetics or climatic factors in either species. In contrast, CHC divergence was strongly associated with an environmental gradient supporting the idea that the climatic environment selects more directly upon these chemical signals. In light of recently advocated models of diversification via ecological selection on secondary sexual traits, the different environmental associations we found for song and CHCs suggest that the impact of ecological selection on population divergence, and how that influences speciation, might be different for acoustic versus chemical signals.

A genes eye view of ontogeny: de novo assembly and profiling of the Gryllus rubens transcriptome

Crickets (Orthoptera:Gryllidae) are widely used model organisms for developmental, evolutionary, neurobiological and behavioural research. Here, we developed a de novo transcriptome from pooled RNA-seq Illumina data spanning seven stages in the life cycle of Gryllus rubens. Approximately 705 Mbp of data was assembled and filtered to form 27 312 transcripts. We were able to annotate 52% of our transcripts using BLAST and assign at least one gene ontology term to 41%. Pooled samples from three different ontogenetic stages were used for transcriptomic profiling revealing patterns of differential gene expression that highlight processes in the different life stages. Embryonic and early instar development was enriched for ecdysteroid metabolism, cytochrome P450s and glutathione production. Late instar development was enriched for regulation of gene expression and many of the genes highly expressed during this stage were involved in conserved developmental signalling pathways suggesting that these developmental pathways are active beyond embryonic development. Adults were enriched for fat transport (mostly relating to egg production) and production of octopamine, an important neurohormone. We also identified genes involved in conserved developmental pathways (Hedgehog, Hippo, Wnt, JAK/STAT, TGF-beta, Notch, and MEK/ERK). This is the first transcriptome spanning ontogeny in Gryllus rubens and a valuable resource for future work on development and evolution in Orthoptera.

Bias in the heritability of preference and its potential impact on the evolution of mate choice

The evolution of mate choice is a function of the heritability of preference. Estimation in the laboratory is typically made by presenting a female with a limited number of males. We show that such an approach produces a downwardly biased estimate, which we term the heritability of choice. When preference is treated as a threshold trait then less biased estimates are obtained particularly for preferences based on the relative value of the preferred trait. Because females in the wild typically survey on average less than five males we argue that the heritability of choice may be more meaningful than the heritability of preference. The restricted number of males surveyed can lead to a reduction in the phenotypic variance of the preferred trait in the group of males selected by the females if the phenotypic variance in preference is equal to or less than the phenotypic variance in the referred trait. If the phenotypic variance in preference exceeds that of the preferred trait then the opposite occurs. A second effect of the restricted number of males sampled is that females are likely to mate initially with males that are not the most preferred. The failure to find the most preferred male may account for the common observation of multiple matings and extra-pair copulations. We suggest that current explanations for polyandry need to take this failure into account.

The potential influence of morphology on the evolutionary divergence of an acoustic signal

The evolution of acoustic behaviour and that of the morphological traits mediating its production are often coupled. Lack of variation in the underlying morphology of signalling traits has the potential to constrain signal evolution. This relationship is particularly likely in field crickets, where males produce acoustic advertisement signals to attract females by stridulating with specialized structures on their forewings. In this study, we characterize the size and geometric shape of the forewings of males from six allopatric populations of the black field cricket (Teleogryllus commodus) known to have divergent advertisement calls. We sample from each of these populations using both wild-caught and common-garden-reared cohorts, allowing us to test for multivariate relationships between wing morphology and call structure. We show that the allometry of shape has diverged across populations. However, there was a surprisingly small amount of covariation between wing shape and call structure within populations. Given the importance of male size for sexual selection in crickets, the divergence we observe among populations has the potential to influence the evolution of advertisement calls in this species.

Evidence for genetic variation in human mate preferences for sexually dimorphic physical traits

Intersexual selection has been proposed as an important force in shaping a number of morphological traits that differ between human populations and/or between the sexes. Important to these accounts is the source of mate preferences for such traits, but this has not been investigated. In a large sample of twins, we assess forced-choice, dichotomous mate preferences for height, skin colour, hair colour and length, chest hair, facial hair, and breast size. Across the traits, identical twins reported more similar preferences than nonidentical twins, suggesting genetic effects. However, the relative magnitude of estimated genetic and environmental effects differed greatly and significantly between different trait preferences, with heritability estimates ranging from zero to 57%.

Socially flexible female choice differs among populations of the Pacific field cricket: geographical variation in the interaction coefficient psi (Ψ)

Indirect genetic effects (IGEs) occur when genes expressed in one individual affect the phenotype of a conspecific. Theoretical models indicate that the evolutionary consequences of IGEs critically depend on the genetic architecture of interacting traits, and on the strength and direction of phenotypic effects arising from social interactions, which can be quantified by the interaction coefficient Ψ. In the context of sexually selected traits, strong positive Ψ tends to exaggerate evolutionary change, whereas negative Ψ impedes sexual trait elaboration. Despite its theoretical importance, whether and how Ψ varies among geographically distinct populations is unknown. Such information is necessary to evaluate the potential for IGEs to contribute to divergence among isolated or semi-isolated populations. Here, we report substantial variation in Ψ for a behavioural trait involved in sexual selection in the field cricket Teleogryllus oceanicus: female choosiness. Both the strength and direction of Ψ varied among geographically isolated populations. Ψ also changed over time. In a contemporary population of crickets from Kauai, experience of male song increased female choosiness. In contrast, experience of male song decreased choosiness in an ancestral population from the same location. This rapid change corroborates studies examining the evolvability of Ψ and demonstrates how interpopulation variation in the interaction coefficient might influence sexual selection and accelerate divergence of traits influenced by IGEs that contribute to reproductive isolation in nascent species or subspecies.

Phenotypic covariance structure and its divergence for acoustic mate attraction signals among four cricket species

The phenotypic variance-covariance matrix (P) describes the multivariate distribution of a population in phenotypic space, providing direct insight into the appropriateness of measured traits within the context of multicollinearity (i.e., do they describe any significant variance that is independent of other traits), and whether trait covariances restrict the combinations of phenotypes available to selection. Given the importance of P, it is therefore surprising that phenotypic covariances are seldom jointly analyzed and that the dimensionality of P has rarely been investigated in a rigorous statistical framework. Here, we used a repeated measures approach to quantify P separately for populations of four cricket species using seven acoustic signaling traits thought to enhance mate attraction. P was of full or almost full dimensionality in all four species, indicating that all traits conveyed some information that was independent of the other traits, and that phenotypic trait covariances do not constrain the combinations of signaling traits available to selection. P also differed significantly among species, although the dominant axis of phenotypic variation (p(max)) was largely shared among three of the species (Acheta domesticus, Gryllus assimilis, G. texensis), but different in the fourth (G. veletis). In G. veletis and A. domesticus, but not G. assimilis and G. texensis, p(max) was correlated with body size, while p(max) was not correlated with residual mass (a condition measure) in any of the species. This study reveals the importance of jointly analyzing phenotypic traits.

Genetically regulated temporal variation of novel courtship elements in the Hawaiian cricket genus Laupala

The Hawaiian cricket genus Laupala (Gryllidae: Trigonidiinae) has undergone rapid and extensive speciation, with divergence in male song and female acoustic preference playing a role in maintaining species boundaries. Recent study of interspecific differences in the diel rhythmicity of singing and mating, suggests that temporal variation in behavior may reduce gene flow between species. In addition, Laupala perform an elaborate and protracted courtship, providing potential for further temporal variation. However, whether these behavioral differences have a genetic basis or result from environmental variation is unknown. We observed courtship and mating in a common garden study of the sympatric species, Laupala cerasina and Laupala paranigra. We document interspecific differences in the onset and duration of courtship, spermatophore production rate, and diel mating rhythmicity. Our study demonstrates a genetic contribution to interspecific behavioral differences, and suggests an evolutionary pathway to the origins of novel timing phenotypes.

Coevolution of male mating signal and female preference during early lineage divergence of the Hawaiian cricket, Laupala cerasina

Sexual selection is a powerful evolutionary force shaping mate choice phenotypes, initiating phenotypic shifts resulting in (or reinforcing) population divergence and speciation when such shifts reduce mating probabilities among divergent populations. In the Hawaiian cricket genus Laupala, pulse rate of male calling song, a conspicuous mating signal, differs among species, potentially behaving as a speciation phenotype. Populations of the widespread species Laupala cerasina show variation in pulse rate. We document the degree of population differentiation in three features of calling song: pulse rate, pulse duration, and carrier frequency. All show significant population differentiation, with pulse rate showing the greatest heterogeneity. A Mantel test found no relationship between geographic distance and pulse rate divergence, indicating that a simple model of greater divergence with increasing distance cannot explain the observed pattern of differentiation. We demonstrate that female preference functions for pulse rate are unimodal, and that preference means show significant differentiation among populations. Furthermore, estimates of pulse rate preference correlate significantly with mean pulse rates across populations, indicating song and preference coevolve in a stepwise manner. This correlated divergence between signal and preference suggests that sexual selection facilitates the establishment of sexual isolation, reduced gene flow, and population differentiation, prerequisites for speciation.

Heritability of and early environment effects on variation in mating preferences

Many species show substantial between-individual variation in mating preferences, but studying the causes of such variation remains a challenge. For example, the relative importance of heritable variation versus shared early environment effects (like sexual imprinting) on mating preferences has never been quantified in a population of animals. Here, we estimate the heritability of and early rearing effects on mate choice decisions in zebra finches based on the similarity of choices between pairs of genetic sisters raised apart and pairs of unrelated foster sisters. We found a low and nonsignificant heritability of preferences and no significant shared early rearing effects. A literature review shows that a low heritability of preferences is rather typical, whereas empirical tests for the relevance of sexual imprinting within populations are currently limited to very few studies. Although effects on preference functions (i.e., which male to prefer) were weak, we found strong individual consistency in choice behavior and part of this variation was heritable. It seems likely that variation in choice behavior (choosiness, responsiveness, sampling behavior) would produce patterns of nonrandom mating and this might be the more important source of between-individual differences in mating patterns.

Acoustic experience shapes female mate choice in field crickets

Female choice can drive the evolution of extravagant male traits. In invertebrates, the influence of prior social experience on female choice has only recently been considered. To better understand the evolutionary implications of experience-mediated plasticity in female choice, we investigated the effect of acoustic experience during rearing on female responsiveness to male song in the field cricket Teleogryllus oceanicus. Acoustic experience has unique biological relevance in this species: a morphological mutation has rendered over 90 per cent of males on the Hawaiian island of Kauai silent in fewer than 20 generations, impeding females' abilities to locate potential mates. Females reared in silent conditions mimicking Kauai were less discriminating of male calling song and more responsive to playbacks, compared with females that experienced song during rearing. Our results to our knowledge, are the first demonstration of long-term effects of acoustic experience in an arthropod, and suggest that female T. oceanicus may be able to compensate for the reduced availability of long-range male sexual signals by increasing their responsiveness to the few remaining signallers. Understanding the adaptive significance of experience-mediated plasticity in female choice provides insight into processes that facilitate rapid evolutionary change and shape sexual selection pressure in natural populations.